Interferon has been shown to have a profound effect on tumor growth, viral replication, and the immune responses. We have identified two new lymphokine activities in mitogen-stimulated mouse spleen cell supernatant fluids which modulate the antiviral activity of interferon. One is a potentiator which enhances the antiviral activity of interferon. A second is an inhibitor which blocks both the antiviral and the immunoregulatory activity of interferon. The crude preparations also contain immune interferon. The potentiator, and immune interferon are not produced by unstimulated mouse spleen cells. We have developed large scale methods of production of potentiator, inhibitor, and immune interferon. In addition, we have purified the inhibitor away from potentiator and immune interferon by sequential chromatography on BSA-Affi-Gel and Ultrogel columns. Such inhibitor preparations contain 0.019 mg protein/ml, have been purified about 1000-fold, and can negate the antiviral activity of at least 400 units of interferon. The inhibitor prevents the establishment of the interferon induced antiviral state when added as late as 3 hr after interferon, but it does not reverse an established antiviral state. The potentiator has been purified about 200-fold but has not yet been separated from immune interferon. The potentiator enhances the antiviral activity of added fibroblast interferon by factor of 10. The potentiation factor is either immune interferon itself or a factor which comigrates with immune interferon through a gel filtration column. This study will employ inhibitor and potentiator produced and purified in bulk quantities so they may be characterized and employed to study their modulation of the antitumor activity of fibroblast interferon. Information gained from the potentiator studies may lead to an increased efficacy of interferon treatment in the management of tumors. Similarly inhibitor studies may explain the need for the tremendous dosage levels of interferon required for tumor management and lead to a more effective interferon therapy. Further, studies of their roles in the modulation of the antiviral state and in the modulation of the immune response will be undertaken.